1. Verapamil has a negative inotropic effect because it inhibits the influx of Ca²⁺ into the cardiac myocytes. Without Ca²⁺, the myocardium cannot contract.

2. Massaging this point stimulates the baroreceptors in the carotid sinus, mimicking the effect of hypertension. This sends signals to the medulla oblongata that result in vagal (inhibitory) stimulation of the heart, slowing it down. (Be aware, though, that this procedure is very dangerous and must never be attempted without medical training.)

3. Because the sarcoplasmic reticulum of cardiac muscle is so scanty, most of the calcium needed for contraction must come from the extracellular fluid. Thus, the cardiac sarcolemma has calcium channels. Skeletal muscle fibers do not need these channels because their sarcoplasmic reticulum provides adequate calcium for contraction.

4. The blood pressure (afterload) in the pulmonary trunk is much lower than that of the aorta, so it produces less opposition to the ejection of blood from the right ventricle. Thus, even though the right ventricle lags slightly behind the left in the onset of contraction, it is the first to expel blood.

5. Your sketches should resemble figures 13.10 (for a nerve fiber) and 19.14 (for a cardiac myocyte). The major difference between the two is that the action potential of a nerve fiber exhibits no plateau. The reasons for the plateau in cardiac muscle are that (a) Ca²⁺ enters the cell through slow calcium channels, which remain open long after the Na⁺ channels close and thus prolong muscle contraction; and (b) K⁺ channels are closed during the plateau, preventing K⁺ from leaving the cell and thus delaying the cell's repolarization.

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